jonathan7,
I'll try and answer your questions from the information I've gathered over the years however I must say I'm not in the military yet so there will be others who are more informed than myself.
1) You can try as hard as you want but if you don't have the required skills you won't get in, it's as simple as that. The military roles are very much skill based rather than qualification based so you could have a Phd and still fail.
2) No, usually you can suggest a preference but it will be up to the military which aircraft you fly depending on operational requirements, so if they are short of aircrew for a particular aircraft and you meet their requirements they will probably put you on that aircraft.
3) Can't answer, what I will say is that joining the military is a life style rather than a career. You're going to be working in some highly stressed situations with some of the best people, you need to be willing and motivated to work as hard as you can or else you'll be out of the door.
4) I would say yes and probably the world also, it's something you would need to get used to.
5) Your local careers advisor would be the best to answer that question however what you should be looking at are subjects that you're good at, doing Maths, Physics and English would be ideal.
Work hard at school to get the best grades possible, get fit and then even fitter, learn about current affairs and what the Aussis are up to across the globe, get involved with organisations such as the military cadet forces and voluntary organisations.